In recent years, a double pipe has become popular in a wide variety of industrial fields in order to make a compact fluid flowing passage and various kinds of improvement have been carried out for a relative arrangement structure of an outer pipe and an inner pipe.
In the prior art, as a double pipe, a unit described in Patent Document 1, for example, has been known.
In the Patent Document 1 is described a double pipe in which an outer pipe and an inner pipe are integrally formed to each other through a metal bending work.
The double pipe described in the Patent Document 1 is made in such a way that each part of an outer pipe and an inner pipe is partially made common to reduce an amount of manufacturing material and its manufacturing cost is less-expensive. In addition, a common part of the outer pipe and the inner pipe is punched to enable a direct branched part connection in a direction perpendicular to an axial direction to be carried out at the inner pipe.